Triggering of Methane Production in Rice Soils by Root Exudates

Elevated CH4 production in flooded soils during the reproductive growth stages of lowland rice (Oryza sativa L.) is believed to result from decomposition of root exudates and autolysed root tissue. Little else is known about the factors of late‐season CH4 production. This laboratory study investigates the effects of soil properties and crop management practices on CH4 production from rice root exudates. In two anaerobic incubation experiments of 20‐d duration, CH4 production was measured following addition of root exudates and glucose to Philippine rice soils that were collected from (Exp. I) five farmers' fields and (Exp. II) four field treatments that varied in degree of soil aeration through crop rotation and timing of crop residue incorporation. The conversion of glucose‐C to CH4 was 1.6 to 3.6 times greater than the conversion of root exudate C. In Exp. I, rates of CH4 production differed among the five rice soils. The sole soil property that was correlated with cumulative CH4 production was cation‐exchange capacity (CEC). In Exp. II, however, no soil property was correlated with CH4 production from glucose and root exudates. Instead, CH4 production was greatest in the soils that had been sampled from the better‐aerated field treatments, which opposes the common association of CH4 production with anaerobic conditions. The exact reason for this trend is unknown. One possible explanation is that organic matter in soils of the better‐aerated field treatments provided less chemical stabilization of the amended substances, enabling their faster conversion into CH4 Soil properties alone appear inadequate to explain differences in CH4 production from root exudates; crop management practices appear to play a role.